Many proposals have been made for logging while drilling systems as shown by the following examples: Karcher, U.S. Pat. No. 2,096,279 proposes a system utilizing electrical conductors inside the drill pipe. Heilhecker, U.S. Pat. No. 3,825,078 proposes a system utilizing extendable loops of wire inside the drill pipe. Silverman, U.S. Pat. No. 2,354,887 proposes a system utilizing inductive coupling of a coil or coils with the drill pipe near the drill bit with measurement of the induced electrical potential at the earth's surface. Arps, U.S. Pat. No. 2,787,759 and Claycomb, U.S. Pat. No. 3,488,629 propose systems in which pulsed restrictions to the drilling mud flow produces pressure pulse signals at the earth's surface. Also, Godbey, U.S. Pat. No. 3,309,656 proposes a turbine like system which produces repetitive pressure waves which are representative of a measured piece of information. The foregoing prior art patents are incorporated herein by references.
Each of such proposals which has been put in field use has had some drawback of sufficient consequence to prevent its commercial acceptance. One drawback is the inconvenience and time involved for the large number of connections and disconnections of electrical connectors in systems such as proposed by Karcher. Though an induced potential measurement system such as proposed by Silverman may be considered operable for a very short distance, the signal to noise ratio of such a system would prohibit its use as a practical matter. A most significant drawback in such prior art systems is the requirement to provide an electrical power source with a sensing and signal transmitting unit located near the drill bit. Excessive signal attenuation is also a significant drawback in some such systems.
The environment is very hostile at the bottom of a well during drilling. Drill bit and drill collar vibrations may be in the order of 50 g. The temperature is frequently as much as 400.degree. and higher. The bottom hole pressure can be more than 10,000 psi. The drilling fluid flowing through the drill collars and drill bit may be highly abrasive. With present drilling equipment including improved drill bits, the continued drilling time with a particular bit can be in the order of 100 - 300 hours and sometimes longer before it becomes necessary to change the drill bit. Accordingly, a downhole formation condition sensing and signal transmitting unit mounted near the drill bit must be capable of operating unattended for long periods of time without adjustment and with a continuing source of electrical power. Also, the signal communication apparatus must be capable of transmitting a continuing usable signal or signals to the earth's surface while additional joints of drill pipe are conventionally added as usual to the drilling string as the drilled bore hole is increased in depth.
A major problem is the provision of an electrical power source for the downhole sensing and transmitting unit at the bottom of the well with communications apparatus which does not employ an electrical conductor or cable extending from the earth's surface to the downhole unit as proposed in the previously referenced prior art. The reason for this problem is the high temperatures encountered near the drill bit and, to some extent, the severe vibration encountered. Present state of the art electrical batteries and fuel cells are not suitable for producing the necessary voltage and power requirements at the high temperatures encountered and for the extended operating time of a downhole sensing and signal unit. Turbine electrical generators driven by the drilling mud have been tried with some success but the turbine blades and bearings are quickly damaged by the abrasive mud. Also, the high temperatures and high pressures can cause the seals between the turbine and the electrical apparatus chamber to fail, sometimes after only a few hours of operation.